If they test more than one variable, they wouldn't know if it was both variables or one variable that was responsible for any observed effect.
Answer:
Passice immunity
Explanation:
Breast milk, produced by female mammals (including humans), besides being rich in fat and protein is full of IgA. IgA is the immunoglobulin (immune system signal proteins) that is present in body secretions, such as milk.
The baby that is fed from the mother's breast ingests these defense proteins that allow it to tolerate different pathogens and toxins.
It is considered passive immunity, as it does not generate memory and depends on a continuous dose to maintain defense levels.
This goes back to Darwin's theory of Evolution. This why certain animals are different colors. This is to protect them from natural predators so the population stays high or also why giraffes have long necks so they can reach food up on trees.